EFFECTS OF DIFFERENT ELECTROLYTE SYSTEMS ON THE FORMATION OF MICRO-ARC OXIDATION CERAMIC COATINGS OF 6061 ALUMINUM ALLOY

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1 16 Rev. Adv. Mater. Sci. (01) Y.J. Liu, EFFECTS OF DIFFERENT ELECTROLYTE SYSTEMS ON THE FORMATION OF MICRO-ARC OXIDATION CERAMIC COATINGS OF 6061 ALUMINUM ALLOY Y.J. Liu 1, J.Y. Xu1, W. Lin, C. Gao 1, J.C. Zhang 1 and X.H. Chen 1 1 School of Mechanical and Electrical Engineering, Guilin University of Electronic Technology, Guilin 5100 Guangxi Youjiang Bureau of Mines, Tiandong 51501, China Received: October 17, 011 Abstract: Ceramic coatings on 6061 aluminum alloy were prepared by micro-arc oxidation in Na Si, NaAlO, and Na(P systems. The breakdown voltage, thickness and hardness of ceramic coatings in different systems were comparatively studied. The surface morphology and phase composition of ceramic coatings were analyzed using SEM and XRD techniques. The results show that the breakdown voltage is decreased with increasing of concentration, and in Na Si system it is the lowest, NaAlO takes the second place and Na (P is the highest. In constant-current model, the thickness of ceramic coatings shows linear increase with the increment of oxidation time in systems, growth rate remains essentially constant. The ceramic coatings in different systems are mainly composed of -Al and -Al, the relative content of -Al and -Al in NaAlO system is higher than that in the other systems. The hardness of dense layer is relatively high, the hardness of loose layer is rather low. The distance from substrate interface is 15 um, hardness reaches the highest (170 HV), then it begins to reduce gradually in NaAlO 1. INTRODUCTION Micro-arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), can in-situ form the ceramic coatings on the surface of valve metal such as Al, Mg, Ti, and their alloys. Ceramic coatings of MAO are much compacter and harder compared with common surface modification technique, which can dramatically improve the wear resistance, corrosion resistance, heat shock resistance, insulation and other properties of the surface of aluminum alloy [1-]. The main factors affecting MAO process are energy parameters (current density, voltage, pulse frequency and duty cycle),, substrate material and working temperature, etc. Electrolyte components are important parameters of MAO process. Acidic and alkaline Corresponding author: Y.J. Liu, Liuyajuan005@16.com systems are mainly adopted at present. Acidic systems mainly include sulfuric acid, phosphoric acid and its salt solution, etc., at present, they are rarely used due to environmental pollution. Alkaline systems have almost no pollution on environment. The metal ions of anodic reaction can easily turn into a negatively charged colloidal particles which are re-utilized, the other metal particles in the easily convert into negatively charged colloidal particles, then enter into the ceramic coatings to change the phase composition and phase structure of ceramic coatings, thus affect the properties of ceramic coatings. The adsorption degree of ceramic coatings is in sequence of SiO > PO > VO > MoO > WO > B O > CrO [] 7. The breakdown voltage, surface and cross section morphology,

2 Effects of different systems on the formation of micro-arc oxidation ceramic Table 1. Chemical composition of 6061 aluminum alloy. Chemical Si Mg Mn Fe Zn Ti Cr Cu Bal elements Contents ( %) Al Fig. 1. The changing regularity of breakdown voltage with the variation of s concentration. Fig.. The changing regularity of breakdown voltage with the variation of current density. growth rate, cross-section hardness and phase composition were researched in three different alkaline systems in this paper. Effects of different systems on the formation of microarc oxidation ceramic coatings of 6061 aluminum alloy were analyzed. The formulas were further optimized.. EXPERIMENTAL METHOD Experimental samples are 6061 aluminum alloy (15 shown in Table 1. Technological process: wire cutting sample - drilling - polishing on abrasive paper (600 #, 800 # and 1500 #) - washing by pure water - drying - microarc oxidation - washing by pure water - drying - testing properties of ceramic coatings. Main components of three systems are Na Si, Na (P, and NaAlO, adding appropriate NaOH, C 10 H 1 N O 8 Na and other auxiliary additives to adjust the ph value, stability and electrical conductivity of solutions. Power supply used in tests was MAO-65D pulse power supply, which could export asymmetric square wave pulse. The thickness of ceramic coatings was measured by TT60 eddy current thickness meter. The surface morphology of ceramic coatings was characterized by JSM-5610LV scanning electron microscopy (SEM).The phase of ceramic coatings was analyzed by D8 ADVANCE X-ray diffraction (XRD) with diffraction angle : 0~80. The hardness of cross section was measured by HV-1000 microhardness tester.. RESULTS AND DISCUSSION.1. Influencing factors of breakdown voltage The changing regularity of the breakdown voltage with the variation of concentration s is shown in Fig. 1. The breakdown voltage is decreased with the increase of concentration s. Electrical conductivity is increased with the increase of concentration of s, required arcing energy is reduced, so that breakdown voltage is decreased. Different components of have different effects on formation of ceramic coatings, the breakdown voltage in Na Si system is the lowest, NaAlO takes the second place and Na (P is the highest, which was in accordance with Wu [5] researching conclusions. The changing regularity of breakdown voltage with the variation of

3 18 Y.J. Liu, Fig.. The surface morphology of ceramic coatings in Na Si Fig.. The surface morphology of ceramic coatings in (NaP Fig. 5.The surface morphology of ceramic coatings in NaAlO current density is shown in Fig.. The breakdown voltage of three kinds of systems is almost invariant, in the case of certain concentration, there is no significant relationship between breakdown voltage and current density. Yerokhin et al. [6,7] further researches show that breakdown voltage is related to distance between anode and cathode, electrode material and electrode polarity, in addition, no relationship is found out with temperature... Analysis of the surface morphology of ceramic coatings with different systems Samples are treated with three different systems by constant current (8 A/dm ) for 0 and 0 minutes. The changing regularity of surface morphology of ceramic coatings is shown in Fig.. From the macro point of view, ceramic coatings in three systems are all offwhite. From the microscopic point of view, dark spots cover with on the surface sample, this phenomenon shows that the discharge channel exists at that location. Many melting particles appear on the surface of ceramic coatings in Na Si system (as shown in Fig. ). There are no significant melting particles on the surface of ceramic coatings in the other two systems, but show more regular round pie structure (as shown in Figs. and 5).With increase of oxidation time, the microporous aperture of surface largens, the size of discharge channels are significantly increased, their number is decreased, there is molten material with rapid cooling and solidification around the discharge channels... The growth of ceramic coatings in different systems The main concentration of s is all 8 g/l, samples are treated with constant current density (8 A/dm ). The changing regularity of thickness of ceramic coatings with variation of oxidation time is

4 Effects of different systems on the formation of micro-arc oxidation ceramic Fig. 6. The changing regularity of thickness of ceramic coatings with variation of oxidation time. Fig. 7. The changing regularity of hardness of dense layer in different systems. shown in Fig. 6. In the same oxidation time (<80 min), the thickness in Na Si system is the thickest, NaAlO takes the second place, Na(P is the thinnest. Under conditions of certain current density and concentration, the thickness of ceramic coatings in three systems all basically appears linear growth trend with time increasing. Calculation of growth rate is used by the ratio of thickness and oxidation time, as can be seen from Fig. 6, the slope of the line is basically invariant. In the constant current model, the working voltage increases with the thickness ceramic coatings increasing, and the total current density is invariant to generate micro-arc discharge, so that the average growth rate of ceramic coatings is basically invariant in some oxidation time... The changing regularity of hardness of ceramic coatings in different systems The load of micro hardness is 0.98 N, the pressure is kept for 10 seconds to the hardness of dense layer. The changing regularity of hardness of dense layer in different systems is shown in Fig. 7. With the increase of oxidation time, the thickness of coatings is increased, the hardness of dense layer is improved gradually. The hardness of dense layer is the highest in NaAlO system, Na Si takes the second place, Na (P is the lowest. The hardness distribution of cross section coatings is shown in Figure 8. The thickness of ceramic coatings obtained in NaAlO system is 60 um. As can be seen from Fig. 8, the hardness of dense layer is relatively high, the Fig. 8. The hardness distribution of cross section coatings in NaAlO hardness of loose layer is rather low. When the distance from substrate interface is 15 um, the hardness reaches the highest (170 HV), then it begins to reduce gradually. There is a transition zone between the coatings and near the substrate interface with its own substrate material and ceramic coatings material, therefore the hardness is rather low in this region, which further promotes metallurgical bonding between ceramic coatings and substrate [8]. Spectra of XRD of ceramic coatings formed in three different systems is shown in Fig. 9. There is mainly Al substrate and Al diffraction peaks, ceramic coatings mainly consist of the phases of -Al and -Al. -Al phase increases gradually from inside to outside, -Al shows a contrary tendency. -Al phase is thermodynamically stable phase, -Al for the thermodynamics of non-stable phase, and can be

5 10 Y.J. Liu, constant. In the same oxidation time (<80 min), the thickness in Na Si system is the thickest, NaAlO takes the second place, Na (P is the thinnest. () The ceramic coatings in different systems are mainly composed of -Al and -Al, the relative content of -Al and -Al in NaAlO system is higher than that in the other systems. The hardness of dense layer is relatively high, the hardness of loose layer is rather low. When the distance from substrate interface is 15 um, the hardness reaches the highest (170 HV), then it begins to reduce gradually. ACKNOWLEDGEMENTS Fig. 9. Spectra of XRD of ceramic coatings formed in three different systems. transformed into -Al phase within the temperature range of 1000 the relatively strong diffraction characteristics of a and g phase, if the distance is nearest between the diffraction peaks, and there is no overlap with other diffraction peak, then the relative content of -Al and -Al can be calculated by the ratio intensity of these diffraction peaks [10]. The relative content of -Al and -Al is 56.% in Na Si system,.6% in Na (P system and 6.8% in NaAlO The hardness of ceramic coatings obtained in NaAlO system is higher than that in the other systems.. CONCLUSIONS (1) The breakdown voltage is decreased with increasing of concentration, there is no significant relationship between breakdown voltage and current density in three kind of systems. Breakdown voltage is closely related to the type and concentration of systems, it is the lowest in Na Si () In constant current model, the thickness of ceramic coatings shows linear increase with the increment of oxidation time in three kinds of systems, growth rate remains essentially This research work is financially supported by the National Natural Science Foundation of China (Grant Number ), Key Scientific and Technological Project of Guangxi (Grant Number ) and the Project of Scientific and Technical Personnel Servicing Enterprise Action (Grant Number 009GJE1001). REFERENCES [1] X. Nie, A. Leyland and H.W. Song // Surface and Coatings Technology (1999) [] J.K. Li and Q.A. Li // Research and application progress of micro-arc oxidation on the alloys 6 (007) 199. [] D.Y. Wang, Q. Dong and C.Z. Chen // Journal of the Chinese Ceramic Society (005) 11. [] Y.F. Jiang, J.M. Li and B.L. Jiang // Surface Technology 0 (001) 8. [5] H.H. Wu, Characterization and property of the microarc oxidation of aluminum and titanium alloys (Chang chun :Jilin University, 00). [6] A.L. Yerokhin, X. Nie and A. Leyland // Surface and Coating Technology 1 (1999) 75. [7] C. Shao, H.Feng and Y.L.Liang // Applied Chemical Industry 5 (006) 78. [8] J. Tian, Z. Luo and S.K. Qi // Surface and Coating Technology 15 (00. [9] V.N. Malyshev // Zashchita Metalov (1996) 665, In Russian. [10] J. Liang, J.F. Zhou and W.M. Liu // Electroplating & Finishing (005.